Compound fenestration assemblies, sometimes referred to as mulled fenestration assemblies, are formed by attaching two or more individual fenestration units (e.g., windows and/or doors) along one or more mull joints to form a combination of windows, doors, or windows and doors, that can be handled and installed as a single assembly, and which give the appearance of being a single assembly. A simple system for joining the component units involves the placing of one or more spacer boards between the units to be joined and installing screws or other fasteners through the frames of the component units, into the one or more spacer boards, to join the units. Other systems for joining the units involve the use of interlocking brackets or other like devices that can be separately installed on the facing surfaces of the frames to be joined and then coupled together to form the compound unit.
Mull joint strength must be sufficient to maintain integrity of the compound fenestration assembly when subjected to, e.g., wind, etc. The need for sufficient mull joint strength may be amplified with an increase in the number of adjacent fenestration units in a given compound fenestration assembly. Furthermore, some compound fenestration assemblies may include factory manufactured mull joints while others may include mull joints which are completed in the field, but which must still provide sufficient strength to resist anticipated wind loads and other forces.